Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide a system and method for thermal control. Merely by way of example, some embodiments of the invention have been applied to light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability.
In systems including light emitting diodes (LEDs), heat dissipation of control chips and/or the systems usually becomes a concern with the increase of forward conducting currents of LEDs and the decrease of the packaging size of the control chips. To prevent a control chip and/or LEDs from being overheated, the control chip often detects the change of the system temperature. If the system temperature increases to a certain level, the control chip usually enters an over-temperature-protection mode and eventually shuts down the system. A temperature control mechanism can be implemented to reduce drive currents of LEDs if the system temperature reaches a threshold so as to prevent the system temperature from continuing to rise.
Power of an LED lighting system (e.g., an LED lamp) is usually determined by as follows:Pd=Vf*If  (1)where Pd represents the power of the LED lamp, Vf represents the voltage of the LED lamp, and If represents the loss current of the LED lamp.
Heat generated by the LED lamp often needs to be dissipated (e.g., through a thermal resistance related to the package of the LED system) so as to keep the LED lamp safe. An ambient temperature (e.g., the temperature outside the LED lamp) may rise with the heat dissipation of the LED lamp, and in turn reduce the heating dissipation of the LED lamp. The LED control system (e.g., a control chip) is inside of the LED lamp, which also includes one or more LEDs. The ambient temperature is related to the power and the heat dissipation of the LED lamp. A difference between a junction temperature of the LED control system and the ambient temperature can be determined as follows:Tj−Ta=Pd*θja  (2)where Tj represents a junction temperature of the LED control system, Ta represents the ambient temperature, and θja represents the thermal resistance related to the package of the LED control system. According to Equation (2), the junction temperature can be sensed to regulate the power delivered to the LED lamp so as to control the temperature inside of the LED lamp for over-heat protection and for prevention of thermal runaway of the LED lamp.
According to the Equations (1) and (2), the temperature of the LED control system can be detected, and the currents of the LEDs can be adjusted to achieve feedback control of the temperature of the LED control system. For example, if a temperature of a control chip increases to a certain level, the control chip adjusts a drive current associated with one or more LEDs to prevent the temperature of the control chip and/or the ambient temperature from continuing to increase.
FIG. 1 is a simplified conventional diagram showing a relationship of a drive current associated with one or more LEDs and a temperature of an LED control system for temperature control. As shown in FIG. 1, the drive current associated with the one or more LEDs keeps at a magnitude (e.g., ILED_NOM) if the temperature of the LED control system is smaller than a temperature threshold (e.g., TBK). If the temperature of the LED control system exceeds the temperature threshold (e.g., TBK), the LED control system decreases the drive current to reduce the temperature of the LED control system. For example, the magnitude of the drive current changes at a negative slope with the temperature of the LED control system. As an example, if the temperature of the LED control system increases to a higher magnitude T0, the LED control system reduces the drive current to a current magnitude ILED_0. If the temperature of the LED control system increases to another magnitude TENDO, the LED control system reduces the drive current to a low magnitude (e.g., 0).
The temperature control mechanism as shown in FIG. 1 has some disadvantages, such as flickering of the LEDs under certain circumstances. Hence it is highly desirable to improve the techniques of temperature control in LED systems.